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PREPARE() SQL Commands PREPARE()
NAME
PREPARE - prepare a statement for execution
SYNOPSIS
PREPARE name [ ( datatype [, ...] ) ] AS statement
DESCRIPTION
PREPARE creates a prepared statement. A prepared statement is a server-side object that
can be used to optimize performance. When the PREPARE statement is executed, the specified
statement is parsed, rewritten, and planned. When an EXECUTE command is subsequently
issued, the prepared statement need only be executed. Thus, the parsing, rewriting, and
planning stages are only performed once, instead of every time the statement is executed.
Prepared statements can take parameters: values that are substituted into the statement
when it is executed. When creating the prepared statement, refer to parameters by posi-
tion, using $1, $2, etc. A corresponding list of parameter data types can optionally be
specified. When a parameter's data type is not specified or is declared as unknown, the
type is inferred from the context in which the parameter is used (if possible). When exe-
cuting the statement, specify the actual values for these parameters in the EXECUTE state-
ment. Refer to EXECUTE [execute(7)] for more information about that.
Prepared statements only last for the duration of the current database session. When the
session ends, the prepared statement is forgotten, so it must be recreated before being
used again. This also means that a single prepared statement cannot be used by multiple
simultaneous database clients; however, each client can create their own prepared state-
ment to use. The prepared statement can be manually cleaned up using the DEALLOCATE [deal-
locate(7)] command.
Prepared statements have the largest performance advantage when a single session is being
used to execute a large number of similar statements. The performance difference will be
particularly significant if the statements are complex to plan or rewrite, for example, if
the query involves a join of many tables or requires the application of several rules. If
the statement is relatively simple to plan and rewrite but relatively expensive to exe-
cute, the performance advantage of prepared statements will be less noticeable.
PARAMETERS
name An arbitrary name given to this particular prepared statement. It must be unique
within a single session and is subsequently used to execute or deallocate a previ-
ously prepared statement.
datatype
The data type of a parameter to the prepared statement. If the data type of a par-
ticular parameter is unspecified or is specified as unknown, it will be inferred
from the context in which the parameter is used. To refer to the parameters in the
prepared statement itself, use $1, $2, etc.
statement
Any SELECT, INSERT, UPDATE, DELETE, or VALUES statement.
NOTES
In some situations, the query plan produced for a prepared statement will be inferior to
the query plan that would have been chosen if the statement had been submitted and exe-
cuted normally. This is because when the statement is planned and the planner attempts to
determine the optimal query plan, the actual values of any parameters specified in the
statement are unavailable. PostgreSQL collects statistics on the distribution of data in
the table, and can use constant values in a statement to make guesses about the likely
result of executing the statement. Since this data is unavailable when planning prepared
statements with parameters, the chosen plan might be suboptimal. To examine the query plan
PostgreSQL has chosen for a prepared statement, use EXPLAIN [explain(7)].
For more information on query planning and the statistics collected by PostgreSQL for that
purpose, see the ANALYZE [analyze(7)] documentation.
You can see all available prepared statements of a session by querying the pg_pre-
pared_statements system view.
EXAMPLES
Create a prepared statement for an INSERT statement, and then execute it:
PREPARE fooplan (int, text, bool, numeric) AS
INSERT INTO foo VALUES($1, $2, $3, $4);
EXECUTE fooplan(1, 'Hunter Valley', 't', 200.00);
Create a prepared statement for a SELECT statement, and then execute it:
PREPARE usrrptplan (int) AS
SELECT * FROM users u, logs l WHERE u.usrid=$1 AND u.usrid=l.usrid
AND l.date = $2;
EXECUTE usrrptplan(1, current_date);
Note that the data type of the second parameter is not specified, so it is inferred from
the context in which $2 is used.
COMPATIBILITY
The SQL standard includes a PREPARE statement, but it is only for use in embedded SQL.
This version of the PREPARE statement also uses a somewhat different syntax.
SEE ALSO
DEALLOCATE [deallocate(7)], EXECUTE [execute(l)]
SQL - Language Statements 2011-09-22 PREPARE()
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On Apache
Under GNU General Public License
2012-05-26 07:29 @38.107.179.239 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)